This invention relates to a continuous, counter current system and method for the freeze concentration of aqueous solutions. More particularly, the invention relates to the freeze concentration of aqueous comestible solutions, such as fruit juices, coffee and cold water soluble tea extracts, milk, wine, beer, vinegar, and the like, utilizing an improved system and method for reducing the environmental viscosity of ice crystals formed during freeze concentration to thereby facilitate the removal of liquor occluded on the ice crystals and promote the growth of ice crystals in the system.
Freeze concentration procedures are based on the principle that ice crystals which are formed upon cooling an aqueous solution to its freezing point, exclude dissolved solutes as they form to provide ice crystals of pure water in a more concentrated liquid. Freeze concentration is used in a number of commercial procedures, both those in which it is desired to recover the ice crystals as the product of the process such as in the desalination of sea water, the treatment of industrial wastes, and the like to produce pure water, and those in which it is desired to increase the concentration of a dilute solution and recover a concentrated liquor from the ice crystals as a product of the process, such as in concentrating solutions or suspensions containing comestibles, such as coffee or tea extracts, citrus juices, fruit juices, milk, wine, beer, vinegar, and the like.
Regardless of whether ice crystals or concentrated liquor is the desired product, freeze concentration systems basically involve the steps of cooling the aqueous solution until water freezes out in the form of ice crystals, separating the ice crystals thus formed from the resulting concentrated liquor, removing occluded liquor from the crystals and removing the ice crystals from the system. In continuous freeze concentration procedures, the solution to be concentrated passes through several cycles or states, becoming progressively more concentrated, until a desired degree of concentration is reached. As ice crystals form in progressively more concentrated solutions, the more concentrated solutions are more viscous and thus more difficult to remove from the ice crystals. Moreover, the rate of ice crystal growth is related inversely to the concentration of the liquid surrounding it, so that when the concentration of the solution is relatively high, the ice crystals produced are relatively small. This further increases the difficulty of removing liquor from the ice crystals, since it is more difficult to remove occluded liquor from small crystals than large crystals.
The removal of the liquor adhering to the surfaces of the ice crystals is a critical step in the efficient operation of all freeze concentration procedures. Generally the most efficient means for removing liquor adhering to the crystals is a wash column in which a packed bed of ice crystals is washed with water to remove liquor occluded on the ice crystals. The efficiency of a wash column is inversely proportional to the viscosity of the adhering liquor, and directly proportional to the square of the effective crystal diameter. Thus, in order to facilitate the removal of occluded liquor from the crystals, it is desirable to reduce the viscosity of the liquid surrounding the ice crystals. One procedure which has been used heretofor for accomplishing this is to introduce the solution to be concentrated between the crystallizer in which the ice crystals are formed and the wash column in which the ice crystals are purified, and thereby create a counter current flow of the solution and ice crystals in the system.
The prior art contains a number of disclosures pertaining to such counter current flow of liquor and ice crystals in a freeze concentration system. For example, Thijssen, H. A. C., "Freeze Concentration", paper presented at Symposium on Advances In Dehydration and Preconcentration, International Union of Food Science and Technology, Selsdon Park, England, Sept., 1973, discloses the principle of countercurrent flow of liquor and ice crystals in a freeze concentration system and describes the advantages to be obtained by introducing liquid feed between the crystallizer and the wash column. Similarly, U.S. Pat. Nos. 2,540,977 to Arnold; 3,402,047 to Shaul, 3,681,932 to Huber et al; 4,338,109 and 4,332,140 to Thijssen et al; 4,406,679 to Wrobel et al; 4,459,144 to Van Pelt et al and 4,557,741 to Van Pelt each disclose a continuous freeze concentration process in which some variation of counter current flow of ice crystals and liquor is utilized. However, each of these prior systems and processes suffer from one or more disadvantages which adversely affect their effectiveness and efficiency in commercial operations.